This invention relates in general to apparatus and methods for evaporating, distilling, freezing or chilling liquids, and more specifically, to an orbital rod drive for use with a vertical heat transfer tube.
Making an ice slurry under mechanical agitation is a common practice in a wide variety of applications ranging from the manufacture of food products such as ice cream and frozen orange juice to the softening of ice on highways with salt to facilitate plowing. A particularly important application is the manufacture of ice in slurry form to be used as a cold storage. Ice slurries are also useful as refrigerants, e.g., to preserve seafood catches on a fishing vessel. Freezing and chilling apparatus and methods are also used in the manufacture of salts, the concentration of various solutions and suspensions, and the purification of water or other fluids.
Cold storage application for air conditioning systems has been urged by the utility industry as a way to transfer the power demand for cooling the air from daytime to the nighttime and thereby smooth out the overall power demand of the entire power distribution system. Freezing of water into ice would release its latent heat and hence can be an effective cold storage. Unfortunately, ice formed over a heat transfer surface tends to stick to the surface and thereby to block the heat transfer property of that surface. This has been found to be a major stumbling block for the wide use of ice for cold storage.
Currently, there are two types of cold storage systems on the market using ice. One is known as the ice harvester type, where a group of ice making machines are installed over an open storage tank. Ice formed periodically to a certain thickness is harvested into the tank by a defrosting cycle. The other one is known as the ice bank type. It employs a group of low cost heat transfer units, usually made of plastic, on which all the ice needed for cold storage accumulates continuously during each chilling cycle. In either of these two types the effectiveness of transferring the heat from the water to the refrigerant during the ice forming process is not as efficient as desired, thus increasing equipment cost.
The concept of making ice in slurry form so that the ice making machine can operate continuously without interruption and with some improved heat transfer property has been attempted in the industry by companies such as the Chicago Bridge and Iron, Inc. and more recently, by the Electric Power Research Institute ("EPRI") with their scheme publicized in the name of "slippery ice". At the present time the performance of the slippery ice cold storage system is still in the evaluation stage.
The EPRI sponsored research to develop a "slippery ice" system was reported in an article entitled "Cool Storage: Saving Money and Energy" published in the July/August 1992 issue of the EPRI Journal. In the EPRI scheme, calcium magnesium acetate, a substance similar to the chemical used for de-icing aircraft, is added to the water. According to EPRI, the use of this additive causes ice to form in the liquid pool, away from the heat exchanger surface, and results in a slushy type of substance that does not cling to metal. The advantages of the "slippery ice" for improving the economy were also reported in Sep. 27, 1992 edition of The New York Times entitled "Keeping Buildings Cool With Greater Efficiency". In this article the use of automobile antifreeze in the water to be frozen was reported to be unsatisfactory because it tends to lower the freezing point too much.
The slippery-ice concept is attractive because it causes an ice slurry to flow down a chilling surface under the influence of gravity only, without mechanical aid. While slippery ice works, how it works is not known. Moreover, this approach has several significant drawbacks. First, only one known additive lets ice overcome the initial stickiness barrier to a gravity feed of crystals down the chilling surface. This is of particular concern where the liquid being processed is a food product; this additive cannot be used. Another limitation is that the heat flux, wetting rate and additive concentration must be carefully controlled for the slippery ice to form. Also, the heat transfer surface must be electropolished.
One of the present applicants has produced evaporation and distillation apparatus and methods which use one or more vertically oriented heat transfer tubes (HTT's) mounted in a container and driven in an orbital motion. These apparatus are described in U.S. Pat. Nos. 4,230,529; 4,441,963; 4,618,399; and 4,762,592. The tubes are smoothed surface, circular in cross section, open at both ends, and made of a material with good heat conductivity properties. A distributor directs a feed liquid to the interior of each tube. The orbital motion spreads the liquid into a film. Heat transferred radially inward through the wall of the tube evaporates a portion of the feed liquid into a vapor stream.
Many known heat transfer apparatus use a rigid wiper bar that is positively driven to rotate within the tube to spread viscous liquids into thin, evenly distributed film. However, any rigid, positively driven wiper or scraper has drawbacks. First there is a need to introduce and seal a rotational drive shaft. Second, because the wiper or scraper is rigid and moving over a fixed surface at close spacings, manufacturing and assembly become difficult and costly. The surface must be machined to close tolerances, as well as the wiper/scraper and its support structures. Further, these rigid arrangements are susceptible to, and comparatively intolerant of, wear.
To solve these problems for low viscosity fluids, e.g. 1 to 1,000 c.p., the '399 patent describes a whip rod located in the tube which spreads the feed liquid into a highly thin and uniform film to reduce its thermal resistance and to enhance its evaporation. The whip rod also controls the build up of solid residue of evaporation. The '399 patent discloses several arrangements for mounting the rod, including lengths of cables, a flexible, but non-rotating anchor connected between a base and the lower end of the rod, and a double universal joint also connected between the lower end of the whip rod and the base. While the whip rod is effective as a film distributor, the mounting arrangements have disadvantages. They increase the overall material, assembly and operating costs. Also, they fail. Material fatigue of flexible cables supporting the whip rods is a particular concern.
While the orbital tube approach has been used for evaporation and distillation, heretofore it has not been applied for freezing. One reason is that city water freezes to the heat transfer surface of an orbital tube evaporator and greatly reduces any performance advantages.
It is therefore a principal object of this invention to provide an apparatus and method for freezing and chilling a process fluid to produce a slurry continuously and at greatly enhanced energy efficiencies.
Another principal object is to provide these results with an apparatus that can be readily scaled up in size.
A further object is to provide a freezer and method of operation that are not limited to any one additive and which can freeze and chill a wide variety of liquids including seawater and food products.
Yet another object is to provide the foregoing advantages without requiring unfavorable restrictions of operating conditions such as heat flux, wetting rate and additive concentration.
A still further object is to provide a freezer and method of freezing with the foregoing advantages that is highly compact.
Another object is to provide the foregoing advantages while also providing favorable capital and operating costs as compared to comparable known equipment and methods.